Throughout this specification, nano size refers to a range of 500 nm or smaller, ultra-fine size refers to a range between 500 nm and 1 μm, and micro size refers to a range of 1 μm or greater.
As materials for main cutting tools or wear-resistant tools, which are necessarily used for metal cutting and other processes in mechanical industries, WC-based hard metals, a variety of TiC or Ti(C,N)-based cermets, other ceramics or high speed steels are used. In general, cermets are ceramic-metal composite sintered bodies including hard phases such as TiC and Ti(C,N), and binding phases such as Ni, Co and Fe as main elements. In addition, the cermets further include additives such as carbide, nitride and carbonitride of Group IVa, Va, and VIa metals of the periodic table. That is, cermets are fabricated in such a way that hard ceramic powder and matrix phase metal for binding the hard ceramic powder are mixed together and then the mixture is sintered in a vacuum, nitrogen or hydrogen ambient. Here, the hard ceramic powder includes WC, NbC, TaC, Mo2C, and so on, in addition to TiC and Ti(C,N), and the matrix phase metal includes Co, Ni, etc.
TiC and Ti(C,N) are excellent high strength materials that have been widely applied to various fields. In particular, since TiC has a very high Vickers hardness of 3,200 kg/m2, a very high melting point of 3,150° C. to 3,250° C., relatively favorable anti-oxidation characteristic up to 700° C., and other superior characteristics such as high wear-resistance, high corrosion resistance, good electron radiation, and light-condensing characteristics, TiC and Ti(C,N) have been widely used for high-speed cutting tools as a substitute for WC—Co alloy.
However, in the case of preparing a cermet using TiC, a binding phase metal such as Ni is used as a liquid metal during a sintering process. In this case, since TiC has a larger wetting angle than WC—Co alloy, TiC grains are rapidly grown, leading to a decrease in toughness of TiC. Notwithstanding, Ford Motor Company mass-produced TiC—Mo2C—Ni cermet for the first time in 1956, which did not have enhanced toughness but was used as a material for a high hardness tool for precision machining, particularly, semi-finishing and finishing.
In 1960's and 1970's, to improve the toughness that was a great weakness of the TiC—Ni cermet system, many attempts have been made to add various kinds of elements thereto; however, these attempts did not yield any notable improvements.
In the 1970's, a method for forming Ti(C,N) having a thermodynamically more stable phase by adding TiN to TIC was introduced, improving toughness to some degree. That is, since Ti(C,N) has a finer microstructure than TiC, the toughness of Ti(C,N) can be improved. In addition, Ti(C,N) is advantageous in improving chemical stability, and mechanical impact resistance. In order to improve the toughness, several additive carbides such as WC, Mo2C, TaC, NbC, etc. , have been used, and new products in the form of Ti(C,N)-M1C-M2C— . . . —Ni/Co are even now being commercialized.
When applying the additive carbide for improving toughness, it can be observed that a general microstructure of TiC or Ti(C,N)-based cermet has a core/rim structure in which a binding phase metal such as Ni and Co surrounds a hard phase of the core/rim structure. A core of the core/rim structure corresponds to TiC or Ti(C,N) undissolved in a liquefied metal binder (e.g., Ni, Co, etc.) during the sintering process, thus giving it a high hardness. Conversely, a rim surrounding the core corresponds to a solid-solution (this is expressed as (Ti, M1, M2 . . . (C, N)) between the core element (i.e., TiC or Ti(C,N)) and an additive carbide, and has high toughness rather than high hardness. As such, by adopting the rim microstructure surrounding the core, the cermet having the core/rim structure solved the toughness problem to some degree, which was an major weakness of a simple cermet system such as TiC—Ni and Ti(C,N)—Ni.